Electromagnetic geophysical surveying of the earth's subsurface includes “controlled source” and “natural source” electromagnetic surveying. Controlled source electromagnetic (CSEM) surveying typically includes imparting an electric field or a magnetic field into subsurface earth formations and measuring resultant changes in electric and/or magnetic fields. For example, resultant changes in electric fields can be determined by measuring voltage differences induced in electrodes or antennas, and magnetometers can be used to measure resultant changes in magnetic fields. Recordings of the induced electric and/or magnetic fields are then used to make inferences about the spatial distribution of conductivity of the earth's subsurface.
Solar energy and lightning cause natural variations in the earth's magnetic field, thereby inducing electric currents (known as telluric currents) under the earth's surface. In a marine environment, natural source (e.g., magneto-telluric or MT) electromagnetic surveying typically includes deploying multi-component ocean bottom receiver stations. Simultaneous measurements of orthogonal components of the electric and magnetic fields are recorded, and the results are used to create a subsurface resistivity model.
CSEM surveying typically includes imparting a substantially continuous, time varying electromagnetic field into subsurface formations. In frequency domain controlled source electromagnetic (f-CSEM) surveying, a time varying electric current is passed through a transmitter antenna, where the current includes one or more selected discrete frequencies. In another technique known as transient controlled source electromagnetic surveying (t-CSEM), electric current is passed through a transmitter at the earth's surface (or near the sea floor) in a manner similar to f-CSEM. The electric current may be direct current (DC). At a selected time, a transient is introduced, typically by switching the electric current on or off, or by reversing its direction. Induced voltages and/or magnetic fields are measured at the earth's surface (or at the surface of a body of water), typically with respect to time over a selected time interval. Alternative switching techniques are possible. Structure of the subsurface is inferred by the time distribution of the induced voltages and/or magnetic fields.
The above described methods for f-CSEM and t-CSEM have been adapted for use in marine environments. Cable-based sensors have been devised for detecting electric and/or magnetic field signals resulting from imparting electric and/or magnetic fields into formations below the bottom of a body of water. Electromagnetic survey systems in which receivers are towed in the water behind a survey vessel, analogous to marine seismic surveys, are desirable as surveys with towed receivers are more efficient that those in which receivers are deployed on a bottom surface of a body of water, and later retrieved after part of the subsurface is surveyed.
A problem arises in towed electromagnetic survey systems in that unavoidable movements of receiver system components result in noise voltages being introduced into conductors carrying voltage signals. For example, pairs of electrodes detecting electric fields produce small voltage signals, sometimes on the order of fractions of a nanovolt. Noise voltages deleteriously impact such small voltage signals, and, if the noise voltages are large enough, can make it impracticable to produce accurate measurements based on the small voltage signals.